1. Field of the Invention
The present invention relates to Coriolis gyros. More particularly, this invention pertains to a method for determination of a zero error in a Coriolis gyro.
2. Description of the Prior Art
Coriolis gyros (also referred to as “vibration gyros”) are in increasing use for navigation. They possess a mass system that is caused to oscillate with the oscillation generally being the superposition of a large number of individual oscillations.
The individual oscillations of the mass system are initially independent of one another and can be referred to abstractly as “resonators”. At least two resonators are required for operation of a vibration gyro: one (the first resonator) is artificially stimulated to oscillate, and this is referred to below as the “stimulating oscillation”. The other (the second resonator) is stimulated to oscillate only when the vibration gyro is moved/rotated. This is because Coriolis forces occur in this case that couple the first resonator to the second resonator, absorb energy from the stimulating oscillation for the first resonator, and transfer it to the read oscillation of the second resonator. The oscillation of the second resonator is referred to below as the “read oscillation”.
In order to determine movements (in particular rotations) of the Coriolis gyro, the read oscillation is tapped off and a corresponding read signal (e.g. the read oscillation tapped-off signal) is investigated to determine whether any changes have occurred in the amplitude of the read oscillation, as they represent a measure of the rotation of the Coriolis gyro.
Coriolis gyros may be implemented as both open-loop and closed-loop systems. In a closed-loop system, the amplitude of the read oscillation is continuously reset to a fixed value (preferably zero) by control loops.
An example of a closed-loop version of a Coriolis gyro will be described below in conjunction FIG. 2, a schematic diagram of a Coriolis gyro in accordance with the prior art. The gyro 1 includes a mass system 2 that can be caused to oscillate and is also referred to below as a “resonator”. (A distinction exists between this expression and the abstract “resonators” term previously employed for individual oscillations of the “real” resonator. As mentioned, the resonator 2 may be considered as a system composed of two “resonators” (a first resonator 3 and a second resonator 4). Each of the first and the second resonators 3, 4 is coupled to a force sensor (not shown) and to a tapping system (not shown). The noise produced by the force sensor and the tapping systems is indicated schematically by Noise1 (reference symbol 5) and Noise2 (reference symbol 6).
The Coriolis gyro 1 includes four control loops. A first control loop controls the stimulating oscillation (that is to say the frequency of the first resonator 3) at a fixed frequency (resonant frequency). It comprises a first demodulator 7, a first low-pass filter 8, a frequency regulator 9, a VCO (voltage controlled oscillator) 10 and a first modulator 11.
A second control loop controls the stimulating oscillation at constant amplitude. It comprises a second demodulator 12, a second low-pass filter 13 and an amplitude regulator 14.
Third and fourth control loops are employed to reset the forces that stimulate the read oscillation. The third control loop includes a third demodulator 15, a third low-pass filter 16, a quadrature regulator 17 and a third modulator 22 while the fourth control loop comprises a fourth demodulator 19, a fourth low-pass filter 20, a rotation rate regulator 21 and a second modulator 18.
The first resonator 3 is stimulated at resonant frequency ω1. The resultant stimulating oscillation is tapped off, phase-demodulated by means of the first demodulator 7, and a demodulated signal component is supplied to the first low-pass filter 8 that removes the sum frequencies. (The tapped-off signal is also referred to below as the stimulating oscillation tapped-off signal.) an output signal from the first low-pass filter 8 is applied to a frequency regulator 9 which controls the VCO 10, as a function of the signal supplied to it, such that the in-phase component essentially tends to zero. The VCO 10 passes a signal to the first modulator 11, which controls a force sensor such that a stimulating force is applied to the first resonator 3. When the in-phase component is zero, the first resonator 3 oscillates at its resonant frequency ω1. (It should be noted that all of the modulators and demodulators are operated on the basis of resonant frequency ω1.)
The stimulating oscillation tapped-off signal is also applied to the second control loop and demodulated by the second demodulator 12. The output of the second demodulator 12 is passed through the second low-pass filter 13 whose output is, in turn, applied to the amplitude regulator 14. The amplitude regulator 14 controls the first modulator 11 in response to this signal and the output of a nominal amplitude sensor 23 to cause the first resonator 3 to oscillate at a constant amplitude (i.e. the stimulating oscillation has constant amplitude).
As mentioned above, Coriolis forces (indicated by the term FC·cos(ω1·t) in FIG. 2) occur on movement/rotation of the Coriolis gyro 1. They couple the first resonator 3 to the second resonator 4, and thus cause the second resonator 4 to oscillate. A resultant read oscillation of frequency ω2 is tapped off and a corresponding read oscillation tapped-off signal (read signal) is supplied to both the third and the fourth control loops. This signal is demodulated in the third control loop by the third demodulator 15, sum frequencies are removed by the third low-pass filter 16, and the low-pass-filtered signal is supplied to the quadrature regulator 17. The output of the quadrature regulator 17 is applied to the third modulator 22 to reset corresponding quadrature components of the read oscillation. Analogously, the read oscillation-tapped-off signal is demodulated in the fourth control loop by the fourth demodulator 19, passed through the fourth low-pass filter 20, and the low-pass-filtered signal then applied to the rotation rate regulator 21 (whose output is proportional to the instantaneous rotation rate, and passed as a rotation rate measurement to a rotation rate output 24) and to the second modulator 18 that resets corresponding rotation rate components of the read oscillation.
A Coriolis gyro 1 as described above may be operated in both double-resonant and non-double-resonant forms. When operated in a double-resonant form, the frequency ω2 of the read oscillation is approximately equal to that of the stimulating oscillation (ω1). In the non-double-resonant case, the frequency ω2 of the read oscillation differs from ω1. In double resonance, the output signal from the fourth low-pass filter 20 contains corresponding information about the rotation rate. In contrast (non-double-resonant case), the output signal from the third low-pass filter 16 contains the rotation rate information. In order to switch between the double-resonant and non-double-resonant operating modes, a doubling switch 25 selectively connects the outputs of the third and the fourth low-pass filter 16, 20 to the rotation rate regulator 21 and the quadrature regulator 17.
The mass system 2 (resonator) generally has two or more natural resonances (i.e. different natural oscillations of the mass system 2 can be stimulated). One of the natural oscillations is artificially produced stimulating oscillation. Another natural oscillation is represented by the read oscillation, which is stimulated by Coriolis forces upon rotation of the Coriolis gyro 1. As a result of mechanical structure and unavoidable manufacturing tolerances, it is impossible to prevent other natural oscillations, in addition to the stimulating oscillation and the read oscillation, of the mass system 2, in some cases far removed from resonance, from also being stimulated. Such undesirably stimulated natural oscillations change the read oscillation tapped-off signal as they are also (at least partially) read with the read oscillation signal tap. The read oscillation tapped-off signal is thus composed of a part caused by Coriolis forces and a part that originates from the stimulation of undesired resonances. The undesirable part causes a zero error of unknown magnitude. In such case it is not possible to differentiate between the two parts when the read oscillation signal is tapped off.